Pre-order traversal

Programs that utilize tree structures need to process nodes[?] in a tree (represented as circles in below diagram). Nodes contain information about an object. For now, let's assume each node contains a letter.

Pre-Order Traversal is a type of Tree Traversal[?] algorithm. Pre-order refers to when the root is processed previous to its two subtrees.

Steps to Preorder Traversal

Given a non-empty tree,

1. Process the root
2. Process the nodes in the left subtree with a recursive call
3. Process the nodes in the right subtree with a recursive call

Given a binary tree PY:

The order would go A,B,D,E,G,C,F

Here is an example of Preorder in C++

 template <class Item>
 int preorder_print(const binary_tree_nodes<Item>* ptr)
 // ptr is a pointer to a node in a binary tree OR null
 // meaning empty tree.
 {
    if (ptr != NULL)
    {
        std::cout << ptr->data() << std::endl;
        preorder_print( ptr->left() );
        preorder_print( ptr->right() );
    }
    return 0;
 }

The same example in Haskell might look like

 data Tree a = ET | Node(a, Tree a, Tree a)

 preorder :: Tree a -> [a]
 preorder ET = []
 preorder (Node (x, left,right)) = x : (preorder left) ++ (preorder right)

Compare: Inorder traversal, Post-order traversal